net: Avoid modulus in skb_tx_hash() for forwarding case.

Based almost entirely upon a patch by Eric Dumazet.

The common case is to have num-tx-queues <= num_rx_queues
and even if num_tx_queues is larger it will not be significantly
larger.

Therefore, a subtraction loop is always going to be faster than
modulus.

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 2 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index 81442957c5c2..3c8073fe970a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1735,8 +1735,12 @@ u16 skb_tx_hash(const struct net_device *dev, const struct sk_buff *skb)
 {
         u32 hash;
 
-        if (skb_rx_queue_recorded(skb))
-                return skb_get_rx_queue(skb) % dev->real_num_tx_queues;
+        if (skb_rx_queue_recorded(skb)) {
+                hash = skb_get_rx_queue(skb);
+                while (unlikely (hash >= dev->real_num_tx_queues))
+                        hash -= dev->real_num_tx_queues;
+                return hash;
+        }
 
         if (skb->sk && skb->sk->sk_hash)
                 hash = skb->sk->sk_hash;